This invention relates to a process for producing wood composite material by combining wood particles with a mixed polymethylene poly(phenylisocyanate)/solid novolac phenolic resin binder composition, followed by molding or compressing the combined wood particles and the binder composition.
Composite materials such as oriented stand board, particle board and flake board are generally produced by blending or spraying lignocellulose materials such as wood flakes, wood fibers, wood particles, wood wafers, strips or strands, pieces of wood or other comminuted lignocellulose materials with a binder composition while the materials are tumbled or agitated in a blender or like apparatus. After blending sufficiently to form a uniform mixture, the materials are formed into a loose mat, which is compressed between heated platens or plates to set the binder and bond the flakes, strands, strips, pieces, etc., together in densified form. Conventional processes are generally carried out at temperatures of from about 120 to 225.degree. C. in the presence of varying amounts of steam generated by liberation of entrained moisture from the wood or lignocellulose materials. These processes also generally require that the moisture content of the lignocellulose material be between about 2 and about 20% by weight, before it is blended with the binder.
Plywood production is accomplished by roll coating, knife coating, curtain coating, or spraying a binder composition onto veneer surfaces. A plurality of veneers are then laid-up to form sheets of required thickness. The mats or sheets are then placed in a heated press and compressed to effect consolidation and curing of the materials into a board.
Binder compositions which have been used in making such composite wood products include phenol formaldehyde resins, urea formaldehyde resins and isocyanates. See, for example, James B. Wilson's paper entitled, "Isocyanate Adhesives as Binders for Composition Board" which was presented at the symposium "Wood Adhesives--Research, Applications and Needs" held in Madison, Wis. on Sep. 23-25, 1980, in which the advantages and disadvantages of each of these different types of binders are discussed.
Isocyanate binders are commercially desirable because they have low water absorption, high adhesive and cohesive strength, flexibility in formulation, versatility with respect to cure temperature and rate, excellent structural properties, the ability to bond with lignocellulosic materials having high water contents, and no formaldehyde emissions. The disadvantages of isocyanates are difficulty in processing due to their high reactivity, adhesion to platens, lack of cold tack, high cost and the need for special storage. U.S. Pat. No. 3,870,665 and German Offenlegungs-schrift No. 2,109,686 disclose the use of polyisocyanates (and catalysts therefor) in the manufacture of plywood, fiberboard, compression molded articles, as well as various technical advantages when used as binders.
It is known to treat cellulosic materials with polymethylene poly(phenyl isocyanates) ("polymeric MDI") to improve the strength of the product. Typically, such treatment involves applying the isocyanate to the material and allowing the isocyanate to cure, either by application of heat and pressure (see, e.g., U.S. Pat. Nos. 3,666,593, 5,008,359, 5,140,086, 5,143,768, and 5,204,176) or at room temperature (see, e.g., U.S. Pat. Nos. 4,617,223 and 5,332,458). While it is possible to allow the polymeric MDI to cure under ambient conditions, residual isocyanate groups remain on the treated products for weeks or even months in some instances. It is also known to utilize toluylene diisocyanate for such purposes.
Isocyanate prepolymers are among the preferred isocyanate materials which have been used in binder compositions to solve various processing problems, particularly adhesion to press platens and high reactivity. U.S. Pat. No. 4,100,328, for example, discloses isocyanate-terminated prepolymers which improve product release from a mold. U.S. Pat. No. 4,609,513 also discloses a process in which an isocyanate-terminated prepolymer binder is used to improve product release. A binder composition in which a particular type of isocyanate prepolymer is used to improve adhesiveness at room temperature is disclosed in U.S. Pat. No. 5,179,143.
A major processing difficulty encountered with isocyanate binders is the rapid reaction of the isocyanate with water present in the lignocellulosic material and any water present in the binder composition itself. One method for minimizing this difficulty is to use only lignocellulosic materials having a low moisture content (i.e., a moisture content of from about 3 to about 8%). This low moisture content is generally achieved by drying the cellulosic raw material to reduce the moisture content. Such drying is, however, expensive and has a significant effect upon the economics of the process. Use of materials having low moisture contents is also disadvantageous because panels made from the dried composite material tend to absorb moisture and swell when used in humid environments.
Another approach to resolving the moisture and isocyanate reactivity problem is disclosed in U.S. Pat. No. 4,546,039. In this disclosed process, lignocellulose-containing raw materials having a moisture content of up to 20% are coated with a prepolymer based on a diphenylmethane diisocyanate mixture. This prepolymer has a free isocyanate group content of about 15 to about 33.6% by weight and a viscosity of from 120 to 1000 mPa.multidot.s at 25.degree. C. This prepolymer is prepared by reacting (1) about 0.05 to about 0.5 hydroxyl equivalents of a polyol having a functionality of from 2 to 8 and a molecular weight of from about 62 to about 2000 with (2) one equivalent of a polyisocyanate mixture containing (a) from 0 to about 50% by weight of polyphenyl polymethylene polyisocyanate and (b) about 50 to about 100% by weight isomer mixture of diphenylmethane diisocyanate containing 10 to 75% by weight of 2,4'-isomer and 25 to 90% by weight of 4,4'-isomer.
U.S. Pat. No. 5,002,713 discloses a method for compression molding articles from lignocellulosic materials having moisture contents of at least 15%, generally from 15 to 40%. In this disclosed method, a catalyst is applied to the lignocellulosic material. A water resistant binder is then applied to the lignocellulose with catalyst and the coated materials are then compression shaped at a temperature of less than 400.degree. F. to form the desired composite article. The catalyst is a tertiary amine, an organometallic catalyst or a mixture thereof. The binder may be a hydrophobic isocyanate such as any of the polymeric diphenylmethane diisocyanates, m- and p-phenylene diisocyanates, chlorophenylene diisocyanates, toluene diisocyanates, toluene triisocyanates, triphenyl-methane triisocyanates, diphenylether-2,4,4'-triisocyanate and polyphenol polyisocyanates. The catalyst is included to ensure that the isocyanate/water reaction is not slowed to such an extent that the pressing time necessary to produce the molded product is significantly increased.
Pressing of wafer board, oriented strand board, and parallel strand lumber using steam injection and a conventional binder such as a urea-formaldehyde resin or a polymeric diphenylmethane diisocyanate (MDI) is known. Examples of such known pressing processes are disclosed in U.S. Pat. Nos. 4,684,489; 4,393,019; 4,850,849; and 4,517,147. These processes yield a product having satisfactory physical properties if the binder is completely cured.
The completeness of binder cure may, of course, be determined by destructive testing of samples which have been permitted to cure for varying amounts of time under the process conditions. The cure time to be used during the production process is determined on the basis of the sample which had completely cured in the least amount of time. The disadvantages of this method are readily apparent. Valuable product is destroyed in the testing. Further, any variation in wood composition, extent of binder dispersion on the wood particles, etc. or processing conditions which would affect the rate of binder cure are not taken into consideration in the above-described method.
Binding compositions comprising urea extended polyisocyanates derived from a combination of a polyisocyanate and urea which is in solution with water, and the process for preparing the binding compositions is disclosed in U.S. Pat. No. 5,128,407. This reference also describes a process for preparing a composite material from comminuted particles or veneers of a lignocellulose material comprising coating the particles or veneers with these binding compositions.
A process for producing compression molded articles of lignocellulose type materials by use of an organic polyisocyanate compound as a binder is disclosed by U.S. Pat. No. 5,744,079. The binders comprise (A) an organic polyisocyanate such as, for example, MDI or PMDI, (B) an aqueous emulsion of a wax having a melting point ranging from 50.degree. C. to 160.degree. C., (C) an organic phosphate ester derivative, and (D) optionally, water.
It has been known that organic polyisocyanate resins have excellent adhesion properties and workability as the adhesive for thermo-compression molded articles such as particle boards and medium-quality fiber boards produced from a lignocellulose type material such as wood chips, wood fibers, and the articles exhibit excellent physical properties. However, the excellent adhesiveness of the organic polyisocyanate resins causes disadvantage that the compression molded article adheres firmly to the contacting metal surface of the heating plate in a continuous or batch thermo-compression process.
To solve the disadvantages of the undesired adhesion to the heating plate, it is required that a releasing agent is preliminarily sprayed onto the heating plate surface to form a releasing layer. Japanese Patent Publication No. 3-21321 discloses a method different from the external releasing agent spray, in which a mixture of an organic polyisocyanate and a mineral wax is sprayed onto the lignocellulose type material prior to thermo-compression molding. Japanese Patent laid open application No. 4-232004 discloses a method of thermo-compression molding of a lignocellulose type material by addition of a neutral ortho-phosphate ester as a compatibilizing agent, the wax and the polyisocyanate.
The large scale industrial manufacture of composite materials which are bonded exclusively with polyisocyanates have previously been limited. The use of some of the polyisocyanates, particularly the better performing isocyanates, such as polymethylene diisocyanate has been limited by their cost. Because of the cost constraints, the level of use of these expensive isocyanates is kept low for a given material. One approach to the use of levels of these isocyanates has involved chain extending the isocyanate with inexpensive extenders.
U.S. Pat. No. 4,944,823 describes a composition for bonding solid lignocellulosic materials. Suitable binder formulations are based on the reactive mixture of an isocyanate and a carbohydrate material. These are both effective and inexpensive, and eliminate health hazards associated with the use of formaldehyde. Carbohydrate materials include, for example, sugars and starches, in the presence or absence of other active materials. These carbohydrates are mixed with a liquid diisocyanate and applied to the wood, which is then pressed to form a composite product.
Binder compositions comprising phenolic resins and polyisocyanates are known and described in, for example, U.S. Pat. Nos. 3,905,934, 4,293,480, 4,602,069, 4,683,252, 5,001,190, 5,101,001 and 5,733,952, and WO 88/03090 and WO 89/07626. These binder compositions are disclosed as being suitable for foundry cores and molds. The materials are typically applied in an organic solvent and cured most often in the presence of gaseous amine vapors.
U.S. Pat. No. 3,905,934 discloses dialkyl phthalate ester solvent systems for phenolic resin-polyisocyanate binder systems. The phenolic resins are preferably benzylic ether resins, including novolac resins. These binder compositions are described as improving the ultimate tensile strength of the resultant foundry core products.
Phenolic resin and polyisocyanate binder systems containing a phosphorus component are set forth in U.S. Pat. Nos. 4,602,069 and 4,683,252. The binder compositions of U.S. Pat. No. 4,602,069 require a phosphorus based acid such as, for example, metaphosphoric, hypophosphoric, orthophosphoric, pyrophosphoric or polyphosphoric acid, or phosphorous, hydrophosphorous or pyrophosphorous acid or an organic derivative of these compounds, and optionally, an acid halide and/or a base. U.S. Pat. No. 4,683,252 describes binder comprising a phenolic resin, a polyisocyanate and an organohalophosphate. Novolacs and resoles are disclosed by both of these patents as suitable phenolic resins.
U.S. Pat. No. 5,001,190 and PCT application WO 88/03090 disclose a process for filling a space within a structure with a polyurethane composite in the presence of water. Suitable polyurethane composites comprise (a) adding a coarse aggregate to the space in the structure to be filled, (b) adding a polyurethane binder to the aggregate, wherein the binder comprises (i) a phenolic resin component comprising a resole phenolic resin and a hydrophobic solvent system, and (ii) a polyisocyanate component comprising an aromatic polyisocyanate and a hydrophobic solvent, and (iii) a urethane promoting catalyst.
Foundry binders based on phenolic resole resins and polyisocyanates are described in U.S. Pat. Nos. 5,101,001 and 5,733,952, and PCT application WO 89/07626. The compositions of U.S. Pat. No. 5,733,952 also comprise an epoxy resin and, preferably, paraffinic oil. Polymerized linseed oil is utilized in the binders of WP 89/07626.
Isocyanates are known to be suitable components for treating cellulosic fiber and wood products. Some processes for this treatment are described in, for example, U.S. Pat. Nos. 5,179,143 and 5,674,568. The binders of U.S. Pat. No. 5,179,143 comprise polyisocyanates, compounds containing at least two isocyanate reactive hydrogen atoms and alkylene carbonates. The binders for modified cellulosic products of U.S. Pat. No. 5,674,568 comprise a polymethylene poly(phenylisocyanate), water, and an organic compound having a hydroxy functionality of from 2 to 8 and a molecular weight of about 60 to 8000 and being selected from the group consisting of ester group-free polyhydric alcohols, polyether polyols and mixtures thereof.
Binders comprising polyisocyanates and phenolic resins are known and described as being suitable for preparing wood composite products by U.S. Pat. Nos. 4,209,433, 4,961,795, and 5,217,665. Suitable phenolic resins disclosed by these references are resole resins. U.S. Pat. No. 4,209,433 requires that the polyisocyanate be added to the wood particles prior to the application of the phenolic resin, thereby producing enhanced adhesive characteristics. The binder compositions of U.S. Pat. No. 4,961,795 may be cured with a curing agent comprising an ester, a lactone or an organic carbonate, which may be moderated by an aliphatic mono- or polyhydric alcohol.
A method of producing waferboard is described by U.S. Pat. No. 5,217,665. This method comprises applying first a liquid phenol formaldehyde resin to the surface of the wafers, then a powdered phenol formaldehyde resin. This is followed by forming layup and pressing at elevated temperature and pressure using steam pressing techniques to consolidate the layup into a board and to set the phenolic resin adhesive.
It is the purpose of this invention to make a mixed adhesive for wood composite manufacture that utilizes the strength of both the polyisocyanate and phenolic resins. These compositions do not contain organic solvents and do not require catalysts to cure. The curing temperatures are lower than that of the phenolic alone. Typically, novolac resins are cured by adding a compound which generates formaldehyde. The compositions of the present invention contain no formaldehyde. The water resistance of the composites is better than that of the phenolic alone. Less polyisocyanate can be used which results in a cost savings, and the tendency of the adhesives to stick to the platens is reduced.